1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching (IPS) liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The long and thin shaped liquid crystal molecules can all have the same definite orientational alignment direction. That alignment direction can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment direction of the liquid crystal molecules. Due to optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
Of the different types of liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, have been the subject of significant research and development because of their high resolution and superiority in displaying moving images. LCD devices have a wide range of applications in office automation (OA) equipment and video units because they are light, thin and have low power consumption. The liquid crystal display panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon the polarity and the amount of applied voltage.
Since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, an electric field induced between them is perpendicular to the lower and upper substrates. However, the LCD devices having this perpendicular electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been developed. IPS-LCD devices include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of an IPS-LCD panel will be provided with reference to FIG. 1.
FIG. 1 is a cross-sectional view illustrating a concept of a related art IPS-LCD panel. As shown in FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper and lower substrates 10 and 20 are often referred to as an array substrate and a color filter substrate, respectively. A common electrode 22 and a pixel electrode 24 are positioned on the lower substrate 20. The common and pixel electrodes 22 and 24 are aligned parallel to each other. A color filter layer (not shown) is positioned on a surface of the upper substrate 10 between the pixel electrode 24 and the common electrode 22 of the lower substrate 20. A voltage applied across the common and pixel electrodes 22 and 24 produces an electric field 26 through the liquid crystal 32. The liquid crystal 32 has a positive dielectric anisotropy, and thus it aligns parallel to the electric field 26.
When no electric field is applied across the common electrode 22 and pixel electrode 24, such as an off-state, the longitudinal axes of the liquid crystal (LC) molecules 32 are parallel and form a definite angle with respect to the common electrode 22 and pixel electrode 24. For example, the longitudinal axes of the LC molecules 32 are arranged parallel with both the common and pixel electrodes 22 and 24. In contrast, when a voltage is applied across the common electrode 22 and pixel electrode 24, such as an on state, an in-plane electric field 26 that is parallel to the surface of the lower substrate 20 is produced because the common and pixel electrodes 22 and 24 are on the lower substrate 20. Accordingly, the LC molecules 32 are re-arranged to bring their longitudinal axes into coincidence with the electric field 26. This results in a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right directions from a line vertical to the IPS-LCD panel, for example.
FIG. 2 is a plan view illustrating one pixel of an array substrate according to the related art IPS-LCD device. As shown, gate lines 40 are transversely arranged and data lines 42 are disposed substantially perpendicular to the gate lines 40. A common line 50 is also transversely arranged in parallel with the gate line 40 and is spaced apart from the gate line 40. The gate line 40, the common line 50 and a pair of the data lines 42 define a pixel region P on the array substrate. A thin film transistor (TFT) is disposed at a corner of the pixel region P near the crossing of the gate and data lines 40 and 42.
In each pixel, three common electrodes 44 extend perpendicularly from the common line 50, and two of the common electrodes 44 are disposed next to the data lines 42, respectively. A pixel connecting line 48 is disposed in parallel next to the gate line 40 and is electrically connected to the TFT T. Pixel electrodes 46 extend perpendicularly from the pixel connecting line 48 toward the common line 50. Each of the pixel electrodes 46 is disposed between two of the common electrodes 44 and in parallel with the data line 42. Each of areas I between the respective common electrodes 44 and the respective pixel electrodes 46 is defined as a block where the liquid crystal molecules are re-arranged by the electric fields. As shown in FIG. 2, there are four blocks in one pixel.
The IPS-LCD device according to the related art in FIG. 2 re-arranges and operates the liquid crystal molecules using the electric field that is parallel with the array substrate. Thus, it can provide a wider viewing angle as compared to the LCD device that use an electric field perpendicular to the array substrate. Some additional modifications to the IPS-LCD device have been developed in order to further increase the viewing angle.
FIG. 3 is a plan view of an array substrate for use in an IPS-LCD device having multiple domains according to the related art. With reference to FIG. 3, some of the detailed explanations previously explained with reference to FIG. 2, will be omitted in order to prevent duplicate explanations. As shown in FIG. 3, a pixel connecting line 58 is disposed over a common line 60. A common electrode 54 and pixel electrode 56 extend from the common and pixel connecting lines 60 and 58, respectively, in an up-and-down direction. Both the common electrode 54 and pixel electrode 56 have a zigzag shape with a plurality of bent portions, but they are parallel to each other and arranged alternately. The zigzag shaped electrodes define multidomains in the pixel regions between the bent portions of the common and pixel electrodes 54 and 56. These zigzag shaped electrodes providing multidomains that improves viewing angle as compared to the straight shaped electrodes of FIG. 2. The IPS-LCD device having the above-mentioned multidomains has a problem of color shifting depending on the viewing angles, because the liquid crystal molecules have long and thin shapes.
As also shown in FIG. 3, the pixel connecting line 58 overlaps the common line 60, so that the overlapped area becomes a storage capacitor CST. More specifically, the pixel connecting line 58 acts as one electrode of the storage capacitor CST, while the overlapped portion of the common line 60 acts as the other electrode of the storage capacitor CST. One of the pixel electrodes 56 is connected to a drain electrode 62 so that all of the pixel electrodes 56 can electrically communicate with the TFT T.
FIG. 4 is a graph illustrating viewing angle properties of the IPS-LCD device having the zigzag structure of FIG. 3. The IPS-LCD device having the zigzag-shaped common and pixel electrodes has the improved viewing angles in directions of ±90 and ±180 degrees, such as right-and-left and up-and-down directions, as illustrated by references IVa and IVb in FIG. 4. However, the viewing angles are degraded in directions of ±45 and ±135 degrees, such as the diagonal directions, as illustrated by references IVc and IVd in FIG. 4. Further, color shift also occurs depending on the viewing angles or directions.
When the voltages applied to the electrodes generate the electric fields between the common and pixel electrodes, the liquid crystal molecules rotate about 45 degrees in accordance with the electric fields. This causes gray inversion due to the rotation of the liquid crystal molecules. In particular, when the IPS-LCD is operated in gray mode, the IPS-LCD produces yellowish color in 45(+45) degrees declination with respect to the liquid crystal polarization because of the optical anisotropy properties of the liquid crystal molecules. The IPS-LCD also produces bluish color in 135(−45) degrees declination with respect to the liquid crystal polarization because of the optical anisotropy properties of the liquid crystal molecules.